O'Malley, a Democrat, signed the measure at a crowded bill-signing ceremony on Thursday. The ceremony was attended by NAACP President Ben Jealous and Kirk Bloodsworth, a one-time Maryland death row inmate who was the first person in the United States freed by DNA evidence.

Maryland is now the 18th state to abolish the death penalty.

The bill will not apply to the five men the state has on death row, but the governor can commute their sentences to life without parole. O'Malley has said he will consider them on a case-by-case basis.

The state's last execution took place in 2005, before O'Malley's tenure.

Supporters of the death penalty could still try to petition the bill to the ballot.

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Lt. Governor Anthony Brown released the following statement:

"We have a responsibility to do everything in our power to hold violent criminals accountable, but the facts prove that the death penalty is racially biased, demonstrably unreliable, and an ineffective deterrent to crime," said Lt. Governor Anthony Brown. "In Maryland, justice will be appropriately severe for horrible crimes while we still remain committed to fairness and equality within our criminal justice system."